We previously demonstrated that Spliceosome Mediated RNA Trans-splicing (SMaRT-TM) could repair mutations in the mRNA and protein that cause Cystic Fibrosis (CF) and can restore partial function in cell culture and in vivo models of this disease. Pre-trans-splicing molecules (PTMs) were developed that can trans-splice with efficiencies as high as 17% of cis-spliced mRNA in vitro, and functional correction of chloride channel activity ranging from 16-22% was achieved in polarized human CF epithelial monolayers and in human/mouse xenografts. (Nature Biotechnology 20: 47-52, 2002). [unreadable] [unreadable] During the development of these PTMs, we observed that these molecules can also trans-splice to nontarget pre-RNAs. Real-time RT-PCR studies of the most efficient CF targeted PTM (CF PTM24) showed that trans-splicing to a single non-target pre-mRNA occurred at approximately 3% of the level to the intended CF target expressed in cultured cells. Recent cloning and sequencing of a random sample of molecules trans-spliced by a different PTM targeting HPV showed that 53% of molecules were correctly trans-spliced following co-transfection of plasmids expressing the target and PTM. Taken together, these results indicate that improved specificity of trans-splicing is needed prior to initiating clinical studies. [unreadable] [unreadable] The objective of this Phase I proposal is to identify a specific CF-targeted PTM through use of a high capacity screen which can simultaneously quantify the specificity and efficiency of trans-splicing reactions. In Phase II we will evaluate the therapeutic potential of the best of these PTMs in vectors that allow efficient delivery to the lung by a clinically relevant route. [unreadable] [unreadable]